Modeling of Tapered Corrugated Graphite-Foam Heat Exchangers

Practical applications of compact heat exchangers, using low-permeability (1.4e-10 m²) high-conductivity (150 W/m-K) graphite-foam for heat transfer enhancement, are often limited by the excessive pressure-drop.

In an earlier study, Norton (2003), using straight-cut corrugations through the entire height of the graphite-foam heat exchanger, identified minimal pressure-drop configurations. For these optimal configurations, pressure-drop reductions of nearly two orders of magnitude were achieved when compared to a full block of graphite foam. With a constant heat flux (12 W/cm²) boundary, the high thermal conductivity of the foam has resulted in heat transfer values that are two orders of magnitude higher than those for state-of-the-art compact heat exchangers. In spite of the significant pressure-drop reductions, the friction factors for the optimal straight-cut configurations were still three orders of magnitude higher relative to those of modern compact heat exchangers. Also, it should be pointed out that in the slotted areas of these geometric configurations, the fluid (water) came directly in contact with the heated surface resulting in undesirable hot spots in the slots.

The present study addresses additional techniques to improve upon the results of Norton. In particular, the technique of tapering the inlet and outlet slots of each optimal straight-cut configuration is considered for additional pressure-drop reductions. Following an extensive two-dimensional parametric study using FEMLAB®, newly optimal tapered configurations were identified where minimal pressure-drop values were further reduced. Compared to their optimal straight-cut counterparts, pressure-drop reductions of nearly 20% at relatively low Reynolds numbers and up to 47% at higher Reynolds numbers are achieved.

The heat transfer problem, with a constant heat flux (12 W/cm²) boundary condition imposed, is three-dimensional and is solved using STAR-CD®. With three- dimensional model simulations, the effects of covering the heated surface of the tapered inlet and outlet slots with a thin layer of foam are analyzed. It was found that with foam in the slots, the Nusselt numbers were nearly twice as large as those obtained without the presence of a foam layer, the hot spots on the heater surface were significantly reduced, and the average temperature of the heater surface was also decreased. The results also showed that there was no additional pressure-drop penalty in adding a thin layer of foam in the slots. In conclusion, the results indicate that while heat transfer performance of the graphite-foam heat exchanger was enhanced significantly with the addition of a thin layer of foam in the slots, the reduced pressure-drop is still too high relative to the state-of-the- art compact heat exchangers.